Electrical Discharge Machining

EDM needs an electrically conductive workpiece

Electrical discharge machining (EDM) removes material with repeated electrical discharges across a small gap. Local heating melts and vaporizes tiny amounts of material. The electrode does not cut by rubbing against the work.

Check conductivity before considering EDM. Hardened tool steel is a candidate; ordinary nonconductive glass or ceramic is not. Hardness is less restrictive than in cutting with a conventional tool, but composition and electrical and thermal properties still affect machining behavior.

Why the fluid and the gap matter

The dielectric fluid normally insulates the gap. A voltage pulse breaks down that insulation locally and permits a discharge. Between pulses, the fluid helps cool the region and flush debris. Debris removal and gap control help keep discharges stable. Sinker EDM commonly uses dielectric oil; wire EDM commonly uses controlled deionized water, though oil-based wire machines also exist.

Use sinker EDM for cavities and wire EDM for through profiles

Sinker, ram or die-sinking EDM feeds a shaped electrode toward the work to make a cavity. The cavity is larger than the electrode because removal extends across the spark gap. Mold cavities and hardened dies are typical uses. Electrode wear must be allowed for.

Wire EDM feeds a thin wire through the cutting zone while controlled motion traces a profile. The wire is continuously renewed. An enclosed opening normally needs a start hole for threading; the wire must pass through the work. A blind pocket calls for another approach, such as sinker EDM.

Wire EDM is useful for intricate die openings and thin features with low cutting force. Internal corners still have a radius: the wire radius plus the working gap limits how tight a corner can be. Specify a small radius rather than a mathematically sharp corner.

Offset the wire center by its radius plus the spark gap

Center-path offset = wire radius + spark gap. Measure this distance from the finished contour toward the material being removed. In a simple straight cut, the full cut width (kerf) is wire diameter plus twice the gap.

For an illustrative radius of 0.120 mm and a one-sided gap of 0.018 mm, the offset is 0.138 mm. These are teaching inputs, not machine settings. A control may apply compensation to a programmed part contour; do not also offset the geometry a second time.

Practice a wire offset

Why one offset does not describe every pass

Discharge energy, flushing and finishing passes change the working gap and surface. Actual compensation comes from the machine's process data and inspection. CHMER's EDM FAQ defines the offset as the wire radius plus the discharge gap, and notes that the gap changes with discharge energy.

Low cutting force does not mean no thermal change

EDM can hold fine features and produce a fine finish with suitable finishing passes, but its surface contains small discharge craters. Melted material can solidify again as a recast layer. Heat can alter nearby material, so surface integrity and any required finishing belong in process selection.

Further reading on EDM surfaces and wire systems

Makino discusses thermal surface effects; Sodick explains wire EDM. Low mechanical force is not a guarantee of zero distortion or an unchanged surface.

Compare Electrochemical Machining. Safety references: Electrical Safety, Machine Hazards and Housekeeping and Chemicals and Safety Data Sheets.